Transconductance is a property of certain electronic circuits which relates to a ratio of a change in the output current generated by a circuit versus the change in input voltage supplied to the circuit. Transconductance may be referred to herein as “GM,” and can be represented mathematically as:
                              G          M                =                              Δ            ⁢                                                  ⁢                          I              OUT                                            Δ            ⁢                                                  ⁢                          V              IN                                                          Eqn        .                                  ⁢        1            
A transconductance circuit may be referred to as a “GM circuit.” Ideally, the transconductance of a GM circuit should remain linear with corresponding input voltage changes. Further, the output current of a GM circuit should track corresponding input voltage changes.
In application, however, linear transconductance and output current tracking is difficult to achieve. A GM circuit is often implemented using a differential pair of transistors as shown in the differential pair transconductance circuit 100 of FIG. 1, but this type of circuit suffers from known disadvantages.
The differential pair circuit 100 includes a pair of transistors Q1, Q2 having common emitter couplings. A current source ISOURCE is coupled to the common emitters of Q1, Q2 to bias the circuit. Output currents IOUTP, IOUTM are obtained from collectors of Q1 and Q2. A differential input signal VIN, is represented by the difference of voltages VINP, VINM, which are applied to the corresponding bases of Q1 and Q2. The circuit 100 generates a differential output current IOUT represented by the difference of output currents IOUTP, IOUTM.
As the input voltages VINP, VINM vary, the differential pair generates corresponding output currents IOUTP, IOUTM, which relate to the input voltages. FIG. 2 is a graph 200 illustrating a simulated transconductance and differential output current IOUT for the differential pair circuit 100 of FIG. 1. The simulated transconductance and differential output current IOUT are normalized for illustrative purposes.
As shown in FIG. 2, as a differential input voltage VIN is applied across the bases of Q1 and Q2 from −160 mV to 160 mV, transconductance (GM) of the circuit 100 is linear only for a small range of differential input voltages near 0V. As the differential input voltage VIN, varies away from 0V, the transconductance varies in a non-linear manner.
Further, the output current IOUT does not track changes of the differential input voltage VIN. Rather, IOUT only tracks changes in the differential input voltage VIN from approximately −20 mV to 20 mV, and then begins to saturate. The output current for the differential pair circuit 100 is limited by the output current from the current source ISOURCE.
The differential pair circuit 100 generates an undesirable output error for input voltages VINP, VINM that are supplied at common mode voltage levels. The error is a consequence of the finite output impedance for the current source ISOURCE. The output error exhibits rectification which also degrades the transconductance linearity for the differential pair circuit 100.
FIG. 3 illustrates a doublet transconductance circuit 300 (referred to as a “doublet circuit” herein). The doublet circuit 300 includes complementary sets of area-offset differential transistor pairs. A first set includes transistors, QU1.1, QU1.2 having a current source IU.1 coupled between emitters of each transistor and a first source potential VSS. A complementary transistor pair QL1.1, QL1.2 have a current source IL.1 coupled between emitters of each transistor and a second source potential VDD. A second set includes transistors QU2.1, QU2.2 having a current source IU.2 coupled between emitters of each transistor and the first source potential VSS. A complementary transistor pair QL2.1, QL2.2 have a current source IL.2 coupled between emitters of each transistor and the second source potential VDD.
A first input voltage VINP is applied to the bases of transistors QU1.1, QU2.1, QL1.1, and QL2.1. A second input voltage VINM is applied to the bases of transistors QU1.2, QU2.2, QL1.2, and QL2.2. Output currents IOUTP.1 and IOUTM.1 are obtained from the collectors of QU1.1-QU2.2 and represent half of an overall output current IOUT1 for the doublet circuit 300. Output currents IOUTP.2 and IOUTM.2 are obtained from the collectors of QL1.1-QL2.2 and represent half of an overall output current IOUT2 for the doublet circuit 300.
The transistors QU1.1-QU2.2 and QL1.1-QL2.2 have area offsets as represented by AOFF: 1 where AOFF corresponds to an offset area factor among the transistors. Transistors QU1.2, QU2.1, QL1.2, and QL2.1 are larger than the other transistors by the offset factor AOFF. When activated, the area offset transistors conduct a correspondingly higher current than the smaller transistors.
FIG. 4 is a graph 400 simulating transconductance for the doublet circuit of FIG. 3 for various area offset factors. As illustrated in FIG. 4, the transconductance for the doublet circuit is flattened or “spread” for various area offset factors including AOFF=4 and AOFF=6. For an area offset factor of AOFF=1, the transconductance is similar to that of the differential pair circuit 100 of FIG. 1. As the area offset is increased to AOFF=4, the transconductance linearity is improved for differential input voltages VIN from approximately −20 mV to 20 mV. As the area offset is increased to AOFF=6, transconductance continues to spread but linearity is degraded.
Although the doublet circuit 300 provides improvements for transconductance linearity, the output current is limited similar to that of the differential pair circuit 100. The output current of the doublet circuit 300 is limited by the currents from the current sources IU.1, IU.2, IL.1, and IL.2.
Accordingly, there is a need in the art for a transconductance circuit that improves transconductance linearity and output current.